This invention relates to an insulation material having improved thermal resistivity. More particularly, this invention relates to improving the capability of the insulation material to block the transfer of radiant heat by distributing the opacifier more uniformly throughout the fibrous matrix.
Insulation products made from fibrous glass filaments are found in many applications, including for example, residential and commercial building insulation, high temperature appliances, heating and air conditioning ducts and hot and cold temperature plumbing. Fibrous glass insulation products are used to block the transfer of heat which can be transferred by various methods including convection, conduction and radiation.
Radiation heat transfer occurs when heat is sent through space and is capable of traveling to an object where the heat can be reflected, absorbed or transmitted. For glass fibrous insulation products at least, most of the energy involved in radiation heat transfer is in the mid-infrared region of the electromagnetic spectrum. Examples of radiation heat transfer include the infrared portion of sunlight traveling from the sun to an object on earth and the transfer of heat from a fireplace across a room. Radiant heat does not need a medium, such as air, water or metal to take place.
Many types of insulation can block a portion of the transfer of heat from radiation by absorbing or reflecting the radiation. Silicate glass fibers absorb strongly in the range 8-12 μm and 16-25 μm, dramatically reducing the amount of heat transferred by radiation in these regions. Radiation at wavelengths longer than about 25 μm does not contribute significantly to heat transfer regardless of the glass properties. However, at wavelengths less than 8 μm, and between 12 and 16 μm, silicate glasses typically provide little blocking to radiation. Although boron is typically added to glass wool insulation, its absorption will block radiation only near 7 μm. The near- to mid-infrared wavelength region between about 2 μm and about 8 μm is particularly important to block for glass fibrous insulation at typical usage temperatures.
The literature discloses that opacifying particulates such as graphite or carbon black may be distributed in insulation products to enhance the absorption and/or reflectance of radiant energy. See, for example, U.S. Pat. No. 4,363,738 to Kummermehr, U.S. Pat. No. 4,692,363 to Reiss, et al., and U.S. Pat. No. 4,762,749 to Schuetz. Schuetz is unique in teaching that an opacifier may be incorporated into an extrudable thermoplastic fiber that can be comingled with a “bulking” thermoplastic fiber to form a fibrous product. Schuetz also mentions the IR wavelength range of 7-24 μm as being important. However, comingling opacified thermoplastic fibers with bulk fibers as a means to reduce the overall heat transfer would not be completely satisfactory for at least two reasons. First, the thermoplastic softens and melts in the manufacturing process and tends to become spherical in shape rather than fibrous. The resulting reduction in surface area eliminates essentially all the thermal benefit. Second, the comingling of two fibers types results in opacification of only a fraction of the fibers present in the insulation, which provides much less thermal benefit than the opacification of substantially all the fibers.
The ability to distribute opacifier particles in fibrous matrices of inorganic mineral fibers has met with even less success. Generally, to improve their loft and handling ability, fibrous glass products are sprayed with a chemical binder composition that binds the glass fibers together. Application of opacifier after binder is applied is impractical as a means to permit the opacifier to distribute into the fibrous matrix. But when binder is applied as a solution—as it typically is—after the opacifier, it tends to wash previously applied opacifier to the nodes or intersections of fibers, where it tends to concentrate. This also fails to provide uniform distribution of opacifier.
It would be advantageous if insulation made from glass fibers could be improved to distribute opacifier more uniformly to block the transfer of heat from radiation more efficiently.